Photo Basics – The ISO Wildcard

We’ve discussed two parts of the exposure triumvirate so far – the aperture and shutter speed.  In this instalment we’ll look at the third piece of the puzzle and that’s the ISO setting.

I think of the ISO setting as the wild card in this equation.  It does’t directly control depth of field or action.  It can help us get the right setting on one of the other two components for the result we want; however.

In most situations, you want to use the lowest ISO setting you can and preferably the native ISO setting for the camera. This is going to give you the best image quality with the lowest noise. Sometimes we want to add ‘grain’ (noise in the digital world) so we’ll use a higher ISO setting on purpose knowing what the trade-off will be in overall image quality. But those instances are typically pretty rare.

When you can’t get the shutter speed and/or aperture you want with a given ISO setting and you’re not willing to compromise on either one, the ‘wild card’ comes into play and you can increase your ISO setting to get you the shutter/aperture combination you’re looking for.

Other times there’s really no choice. Shooting sports, for example, even in a well lit arena with television lights is still a pretty dark environment for the camera and you may not be able to get your shutter speed high enough to get the amount of action stopping power you want even shooting wide open. What to do? Increase the ISO setting. It’s common to shoot, for example, hockey or basketball with an f2.8 lens wide open, a shutter speed of 1/400 or faster and an ISO setting of 1600 or even higher.

Bringing back the very cooperative model that is my hibiscus plant, we can see how increasing the ISO affects noise in the image.

This is a 100% crop of the centre of the flower at ISO 100. It’s somewhat blurred due to the aperture setting used but we can see that there’s really no noise in the image.

ISO 100

Moving to ISO 800, we see there’s quite a bit more noise evident in the image but the image is still quite useable and overall image quality isn’t degraded significantly.

ISO 800

The story begins to change when we move to ISO 1600. Here we see significantly more noise and overall image quality is quite degraded.  We also begin to see a loss of detail. Look at the detail on the end of the stamen. There’s some loss of shape and form as well as some alteration in colour.

ISO 1600

When we move to ISO 3200, which is not in the normal range of the Canon 5D used to take the shots, the story gets even worse.  Noise is much worse. Some elements of the image are beginning to meld together from loss of distinct form and contrast. With film, this loss of form would be referred to as reduced accutance or edge clarity. We’re also beginning to see discolouration of pixels at this setting. This is really an emergency-only ISO setting. In other words if you just have to get the shot and have no other choice, use this ISO setting but avoid it otherwise.

ISO 3200

While digital technology has improved greatly in the years since the 5D was released and while noise is generally less at all ISO settings, the relationship of an increase in noise levels commensurate with an increase in the ISO setting still holds. And although higher ISO settings are more useable now, the best practice is still to use the lowest ISO setting you can in a given situation.

So in conclusion, we can use the ISO setting to help us get the shutter speed and/or aperture we want to achieve the image we want.  We can also use the ISO creatively to add ‘grain’.  The caveat to this is, of course, that as we increase the ISO we reduce overall image quality so be aware of the tradeoff when you’re making your exposure triumvirate decisions.

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